One proposed cooling system includes a cooling circuit to cool an engine down, a radiator that cools down the cooling circuit with a supply of the outside air, a cooling fan that is connected with a fan drive motor via a clutch and feeds the supply of the outside air to the radiator, and a thermistor that detects temperature in the cooling circuit. In this proposed cooling system, the cooling fan is driven in response to a detection signal input from the thermistor (see, for example, Japanese Patent Laid-Open Gazette No. 6-207513). This prior art cooling system couples the cooling fan with the motor via the clutch in response to any abnormality, for example, a short circuit or an open circuit, arising in the thermistor to drive the cooling fan. This enhances the cooling efficiency of the cooling circuit and prevents a temperature rise of the engine to an abnormal level.